The question above explores potential reasons why belief in conspiracy theories is more common among the US right wing than the left wing.
Several right wing conspiracy theories have become household names - QAnon is apparently still a thing and Obama-Birtherism is still not that far behind in the rear-view mirror.
Are there any notable conspiracy theories in the USA that are, or until recently have been, supported predominantly by left-wing voters or politicians? By left-wing, I am especially referring to the mainstream US Democratic Party but other left-wingers or far-left politicians such as Communists may also be included.
For example, if there is a left-wing conspiracy theory that says that Mitt Romney is actually an Egyptian sleeper agent hired by Scottish-Rite Freemasons to privatize Medicare and get laws passed requiring all public school teachers to be card-carrying members of the KKK, that would count.
In response to a comment by DoctorDestructo, if a conspiracy theory is predominantly believed by left-wing voters, then that can count as a left-wing conspiracy even if the subject matter of the conspiracy is not clearly partisan.
As for definitions, I am treating this as a "good subjective" question in the best tradition of Stack Exchange. For example, apply whichever definition of "conspiracy theory" and "left-wing" you feel to be most appropriate based on your experience and knowledge.